Environmental Impact of Biofuels Part 6 pot

Environmental Impact of Biofuels 92 that belowground biomass is more important for C sequestration than aboveground biomass as studies showed that changes in SOC pools positively correlated with the quantity of belowground biomass input but not with input of aboveground biomass (Russell et al. 2009; Lu et al. 2011). Balesdent and Balabane (1996) measured root-derived C in maize cultivated soils and found that although the shoot to root ratio was only 0.5 root- derived C was 1.5 times higher than aboveground-derived C (from stalks and leaves). Furthermore, root litter of grasses is of lower quality and therefore less easily decomposable compared to aboveground litter due to lower N but higher lignin concentration (Vivanco and Austin 2006). This higher recalcitrance of plant litter slows down the litter decay process and increases the amount of C stored in the soil (Sartori et al. 2006; Johnson et al. 2007). 4.2 Biofuel feedstock harvest and global change The sustainability of biofuel feedstock harvest under global change needs to be evaluated in order to quantify changes in the net ecosystem C balance as well as assess a possible positive feedback to climate change. Biofuel feedstock harvest and the coherent changes in the C balance can be evaluated from experimental studies that use clipping or biomass harvesting to remove aboveground biomass (Luo et al. 2009). One study that combined the effects of climate warming and biomass feedstock harvesting on ecosystem C dynamics was conducted in the Southern Great Plains, USA, which is considered to be a major region for biofuel feedstock production (Luo et al. 2009). Temperatures were increased by 2°C and biomass was clipped annually. On average, data of nine years showed increased net primary productivity (NPP) under warming and even higher values in the combination treatment of warming and clipping. Although warming increased soil respiration rates clipping showed a decreasing trend in soil respiration. Yearly biomass removal reduced the C input to soils which was clearly demonstrated by higher losses of soil C in the clipped plots compared to the unclipped plots (Fig. 3). In both clipped treatments losses in soil C after nine years were more than twice as high as they were for the unclipped plots. Additionally, warming enhanced soil C loss resulting in the highest loss of soil C under clipping and warming treatment (Fig. 3). These results clearly show that biofuel feedstock harvest in combination with warmer temperatures results in the highest loss in soil C. control warmed control warmed g C m -2 (9yr) -1 -1800 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0 Unclipped Clipped Fig. 3. Change in soil C content between 1999-2008. Values are means of 5 plots ± 1 se Biofuels and Ecosystem Carbon Balance Under Global Change 93 4.3 Clipping-induced erosion under global change Changes in land use through alteration of land coverage and disturbance of soil structure result in changes in soil moisture which can induce higher soil erosion rates (Lal 2004). Generally, plant coverage protects the soil from soil erosion by intercepting rainfall and runoff. Plant cover, plant height, rooting characteristics and other plant related parameters are important factors in reducing soil erosion rates (Wilhelm et al. 2007; Johnson et al. 2010). If aboveground biomass is removed for biofuel feedstock harvest more bare ground will increase temperatures as well as surface runoff and thus accelerate soil erosion (Schlesinger et al. 1990; Zuazo and Pleguezuelo 2008). Cover and type of vegetation can therefore affect soil erosion and potentially lead to a net source of C by soil erosion induced loss of SOC. Control Warmed Erosion rate (mm yr -1 ) 0 500 1000 1500 2000 2500 Control Warmed Soil C loss (g m -2 yr -1 ) 0 20 40 60 80 ab Fig. 4. a) Yearly erosion rate in the clipped subplots, b) yearly soil C loss in the clipped subplots. Values are means of 16 measurements per treatment ± SD. Redrawn with permission from Global Change Biology Bioenergy, Xue et al. 2011 It is well known that biomass removal on a continuous basis results in increased soil erosion but it is not well known how a warmer climate might amplify C loss from soils through erosion. The only study, we are aware of, that combines the effects of biomass removal and climate warming on soil erosion rates was conducted in a tallgrass prairie in the Southern Great Plains, USA (Xue et al. 2011). In a multiyear experiment (since 1999) grassland was warmed (+2°C) on a whole ecosystem-level and half the plots were clipped in order to mimic biofuel feedstock harvest. One side effect of warming was a reduction in soil moisture which was even greater in the clipped plots. Clipping-induced relative soil erosion rate was threefold increased under the warming scenario (Fig. 4a). These high erosion rates resulted in high losses of SOC (Fig. 4b). The stronger response to the warming treatment in the clipped plots was ascribed to lower soil moisture in the clipped plots as evaporation from the soil surface was increased when biomass was removed. Some of the consequences of higher erosion rates are reduced soil fertility, degraded soil structure and reduced SOC, all being enhanced by biomass removal. The soil that is most affected by erosional processes is the SOC-rich upper soil level making erosion a net source of C to the atmosphere (e.g. Lal 2003). 5. Interactive effects of biofuel feedstock harvesting and global change 5.1 Biofuel feedstock harvesting and NECB Soils and their C stocks will be affected by land use change and by manipulations in the substrate supply but more importantly changes in the soil C budget will potentially affect the net ecosystem C balance (Fargione et al. 2008; Sanderson 2008; Luo et al. 2009) and consequentially contribute to the overall terrestrial C-cycle feedback. Environmental Impact of Biofuels 94 Ecosystems can function as C sources or C sinks and their role in the global C cycle becomes even more important with global change as ecosystems either release or absorb atmospheric CO 2 and with it enhance or mitigate climate warming (Chapin et al. 2006). Net ecosystem production (NEP) is a measure of gross primary productivity (GPP) minus ecosystem respiration and mostly coincides with the net ecosystem C balance (NECB) unless C in other forms than CO 2 or dissolved organic C moves in or out of the system (Chapin et al. 2006; Lovett et al. 2006). Therefore, NECB is the net estimate of C accumulation (positive NECB) or C loss (negative NECB) in any system. If an ecosystem's net C balance is positive the ecosystem functions as a C sink by sequestering C. In contrast, a negative NECB implies C release to the atmosphere and any ecosystem showing a negative balance functions as a C source. NECB can be applied on short-term or long-term scales and to any spatial scale which makes it a very useful parameter for cross-scale comparisons (Chapin et al. 2006). To fully estimate the impact of biofuel feedstock removal on ecosystems under global change the net ecosystem C balance needs to be calculated to estimate a feedback of biomass removal to climate change. So far there are not many studies that measure the impacts of biofuel feedstock harvest on the net ecosystem C balance under global change. Nevertheless this is important as biofuels are supposed to help mitigate climate change by reducing CO 2 release from fossil fuels. But if biofuel feedstock harvest has large negative impacts on the net ecosystem C balance this mitigation strategy might not help reduce CO 2 release to the atmosphere. 5.2 NECB under elevated CO 2 Elevated atmospheric CO 2 generally increases above- as well as belowground biomass and also enhances soil C storage although the extent to which C is stored in soils is largely dependent on N availability (Luo et al. 2006). Belowground biomass often shows a higher response to elevated CO 2 therefore increasing C input to soils (Luo et al. 2006). C accumulation in plant and soil pools reflects increased C input into ecosystems that usually decreases litter quality and with it decomposability. Decreasing decomposability also derives from increased mycorrhizal growth under elevated CO 2 that enhances physical protection through formation of intra-aggregate or organomineral complexes to protect organic matter from microbial decomposition (Rillig 2004). Large fractions of the C accrued in soils under elevated CO 2 derive from increased belowground biomass growth which is not affected by biomass removal. Nevertheless there are some factors that need to be considered when making predictions about net ecosystem C balances for biofuel feedstock harvest under elevated CO 2 . It is not yet clear whether there will be a down-regulation of CO 2 stimulation of photosynthesis and with it in plant growth and other C processes under persistent CO 2 stimulation (Long et al. 2004;). Photosynthetic acclimation was alleviated in grassland when plants were harvested but only under high N availability (Ainsworth et al. 2003). Low N conditions resulted in some acclimation of photosynthetic capacity. It seems that all responses of C processes under elevated CO 2 are strongly dependent on N availability. However, when only considering the global change factor elevated CO 2 , biofuel feedstock harvest might still allow for C sequestration in soils resulting in a positive net ecosystem C balance. 5.3 NECB under climate warming Unlike elevated CO 2 that primarily influences C uptake through photosynthesis warming affects almost all chemical and biological processes. Furthermore, warming involves some secondary effects on ecosystems such as extended growing seasons, change in species Biofuels and Ecosystem Carbon Balance Under Global Change 95 composition and drier conditions. Hence, it is not surprising that ecosystem warming experiments have produced inconsistent results regarding plant growth, soil respiration and net ecosystem production. Nevertheless the most important biomass fraction for C sequestration under biofuel feedstock harvest is the belowground biomass which was positively stimulated under warming and harvesting scenarios (Luo et al. 2009). This positive interaction was ascribed to over-compensatory mechanisms of plant physiological processes to biomass removal (Owensby et al. 2006). As belowground biomass growth is enhanced under warmer conditions the C loss through biomass removal might be less important for the net ecosystem C balance than the gain in C through increased belowground biomass. On the other hand continuous biomass removal increases soil erosion rates (Xue et al. 2011) which is accompanied by high losses of soil C. Even higher erosion rates occur when biomass removal takes place under warmer conditions as the soil dries out more easily leaving unstable soil structures favoring soil erosion. Therefore, biomass harvesting of natural grassland (Luo et al. 2009) in combination with warming resulted in a more negative net ecosystem C balance than for the warming treatment alone (Fig. 5). The more negative C balance is mainly due to high soil C losses (Fig. 4) as C input to soils was smaller than the C lost through CO 2 release and soil erosion. Thus, over- compensatory belowground biomass growth was not enough to offset soil C loss under warming and clipping. This long-term experiment shows that growing biofuel feedstock for harvesting under climate warming puts an additionally strain on the ecosystem C balance and does not help to sequester more C in order to reduce CO 2 release to the atmosphere. Unclipped control warmed control warmed g C m -2 yr -1 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Clipped Fig. 5. Net ecosystem C balance calculated per year for the period of 2000-2008. Values are means of 6 plots ± 1 se 5.4 NECB and change in precipitation Changes in precipitation as a consequence of global change include more frequent extreme precipitation and drought events which likely have large effects on ecosystem processes (Weltzin et al. 2003). Precipitation is an important factor in shaping ecosystem C dynamics as aboveground biomass and soil respiration linearly increase with mean annual precipitation but belowground biomass and soil C content remain rather constant (Zhou et Environmental Impact of Biofuels 96 al. 2009). As was shown for the Southern Great Plains in the USA no change in belowground C allocation is more important to the net ecosystem C balance than higher aboveground plant growth since this higher aboveground litter input was compensated by higher litter decomposition. A more positive net ecosystem C balance therefore seems plausible under wetter conditions. On the other hand warming induced drought suppresses net primary productivity and turns ecosystems into net sources of carbon dioxide (Ciais et al. 2005; Arnone et al. 2008). If additionally biomass is removed the net ecosystem C balance could become even more negative contributing more to a positive carbon-climate feedback. 5.5 NECB and N addition N addition strongly influences ecosystem C processes through photosynthesis and biomass production and therefore has large impacts on the net ecosystem C balance. Generally N addition increases C input to soil through increased aboveground litter input (Liu and Greaver 2010). With higher N availability plants invest less C into belowground biomass as roots can more easily acquire N. Furthermore, higher N availability strongly influences the shoot to root ratio and root litter flux to soil decreases (Liu and Greaver 2010). If additionally C from aboveground biomass is not returned to soil due to biofuel feedstock harvest total C input to the soil will decrease and a negative net ecosystem C balance is very likely. 6. Conclusion Growing biofuels for alternative energy can help mitigate increasing atmospheric CO 2 concentration; however continuous biofuel feedstock harvest will influence the whole ecosystem C balance possibly resulting in a positive feedback to climate change. Ecosystem C processes are strongly influenced by global change factors and their interactive effects are very complex and not yet well understood. An overall response of biomass feedstock removal on the net ecosystem C balance under global change is therefore still speculative but we know that global change factors that enhance root biomass have a more positive effect on the net ecosystem C balance when biomass is continuously removed than factors that enhance aboveground biomass. Increased CO 2 concentration in the atmosphere has the potential to increase belowground C storage especially when N and other nutrients are not limiting. Climate warming on the other hand seems to reduce soil C storage as C decomposition and C losses through soil erosion under biofuel feedstock harvest are higher. Responses to changes in precipitation are very variable but drier conditions result in a more negative ecosystem C balance if biomass is continuously removed. This effect could be neutralized again under elevated CO 2 as stomatal conductance and evapotranspiration decline thus decreasing the plant water use. N availability is a crucial factor for optimized plant growth and C storage but high N addition can also reduce belowground biomass and thus C input to soils. If additionally all biomass is removed there will be an even smaller C input into soil. One way to alleviate strong impacts of biomass harvest on C-cycling might be to harvest at a later time as harvesting after plant senescence showed to reduce C and N losses although biomass yield might be slightly lower (Heaton et al. 2009; Niu et al. 2010). In conclusion, this chapter showed that biofuel harvesting has large impacts on the net ecosystem C balance which are likely enhanced under global change. More information on interactive effects of multiple global change factors is still needed to fully estimate the impacts of biofuel feedstock harvest on net ecosystem C balance and any possible feedback to climate change. Biofuels and Ecosystem Carbon Balance Under Global Change 97 7. References Ainsworth, E. A.; Davey, P. A.; Hymus, G. 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Source components and interannual variability of soil CO 2 efflux under experimental warming and clipping in a grassland ecosystem. Global Change Biology, 13, 4, 761-775 Zuazo, V. H. D. & Pleguezuelo, C. R. R. (2008). Soil-erosion and runoff prevention by plant covers. A review. Agronomy for Sustainable Development, 28, 1, 65-86 [...]... et 40 - 266 0 (1180) 1.7 - 127 (49.5) 1 .6 – 44 .6 (22.7) al., 2002 Zdrahal et 4 46 - 41 06 (20 06) 21 - 259 (1 16) 7 .6 – 61 .5 (31) al., 2002 1182 - 69 00 Simoneit et 6 - 371 (1 26) 2 – 148 (55) (2 460 ) al., 2004 Decesari et 284 - 7485 (2222) 23.7 - 543 (152) 7.7 - 261 (58.7) al., 20 06 Decessari 763 - 7903 ( 369 8) 34.0 - 345 (151) 16. 4 - 193 (80.3) et al., 20 06 Yttri et al., 134 - 971 (407) 34 - 2 86 (1 16) 1 - 7... (66 ) 4.4 – 44.2 (19 .6) al., 2002 Pashynska 420 61 25 et al., 2002 Pashynska 19.1 3 1 et al., 2002 Simoneit et 121 - 1133 (477) 17 - 153 (66 ) 4 - 44 (20) al., 2004 Simoneit et al., 2004 6 - 56 0.2 - 15 0 .6 - 2.4 1 162 - 33400 154 - 4430 Simoneit et (14 460 ) (1422) 84 - 2410 (1014) al., 2004 Simoneit et 1350 108 1 06 al., 2004 Yttri et al., n.d - 475 ( 166 ) n.d - 155 (41) n.d - 17 (3) 2007 Ward et 860 - 60 90... 120 160 80 120 160 80 120 160 (b) 1998 30 Latitude 20 10 0 -10 -80 -40 0 40 (c) 1999 30 Latitude 20 10 0 -10 -80 -40 0 40 Longitude Fig 1 The geographical distribution of fire hot spots in the tropics derived from ATSR data Fig 2 Monthly variability of fire hot spots in the southeast Asian subcontinent (30 °N, 90 °E - 5 °N, 115 °E) 104 Environmental Impact of Biofuels Fig 3 Photos showing storage of. .. compounds with a wide range of chemical and physical properties Recent advances in the 102 Environmental Impact of Biofuels speciation of the OC fraction in smoke aerosol generated from biofuel combustion provide some new insights into the chemical and physical characteristics of such particles For instance, it is now understood that biomass smoke particles contain a sizeable portion of higher molecular weight... 20 06 Yttri et al., 134 - 971 (407) 34 - 2 86 (1 16) 1 - 7 (2) 2007 Yttri et al., 232 - 971 (60 5) 56 - 2 86 ( 167 ) 1.1 - 6. 8 (4.0) 2007 Yttri et al., n.d - 151 (47) n.d - 42 (10) n.d – 7.5 (3) 2007 1182 - 69 00 (2 460 ) 6. 0 - 371 (1 26) 2.3 - 148 (55.4) Biofuel Combustion Emissions - Chemical and Physical Smoke Properties Particle Location Season Size Urban Winter PM10 Urban Winter PM10 Urban Summer PM10 Urban... generation and transportation The utilization of biofuels in such controlled combustion processes has the great benefit of not further depleting the limited resources of fossil fuels, yet it is associated with emissions of greenhouse gases and smoke particles similar to traditional combustion processes, i.e., those of fossil fuels On the other hand, a vast amount of biofuels is subject to combustion in small-scale... (30) 2010 Krumal et 422 ± 165 71.2 ± 25.8 19.5 ± 7 .67 al., 2010 Krumal et 572 ± 71.3 105 ± 14.1 48.7 ± 2.92 al., 2010 Zhang et 15 .6 - 472.9 al., 2010 Sang et al., 2011 26. 2 – 133.7 ( 36. 0) Sang et al., 21.1 – 91.5 (30.0) 2011 Table 2 Ambient concentrations of anhydrosugars reported in the literature 108 Environmental Impact of Biofuels Biomass Combustion Particle Lev/ Location Lev/Man Lev/Gal type type... smoke particles and gases released from uncontrolled biofuel combustion impose significant effects on regional and global climate Estimates have shown the majority of carbonaceous airborne particulate matter to be derived from the combustion of biofuels and biomass The resulting “clouds” of carbonaceous aerosol particles nowadays span vast areas across the Globe Aside from the negative health impacts... identification of open/stove 1 06 Environmental Impact of Biofuels fires are below 0.2 for wood combustion in fire places and ovens, while they approach 0.5 for open fires (Fine et al., 2001; Fine et al., 2002; Fine et al., 2004a; Puxbaum et al., 2007) Fig 4 Typical chemical composition of smoke particles derived from rice straw burning Table 2 gives a summary of ambient concentrations of levoglucosan, mannosan... fraction of levoglucosan present in a super-coarse mode (>10 μm aerodynamic particle diameter) as well as a fine mode ( . - 69 00 (2 460 ) 6. 0 - 371 (1 26) 2.3 - 148 (55.4) Graham et al., 2002 Rural Dry PM 2.5 40 - 266 0 (1180) 1.7 - 127 (49.5) 1 .6 – 44 .6 (22.7) Graham et al., 2002 Rural Dry PM 2.5 4 46 - 41 06. meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia, 1 26, 4, 543- 562 Environmental Impact of Biofuels. Impact of Biofuels 102 speciation of the OC fraction in smoke aerosol generated from biofuel combustion provide some new insights into the chemical and physical characteristics of such particles.